_to_string: Fix the bug when a 2-D sequence is passed

[seisman]

Patches #3986.

When a 2-D sequence is passed to _to_string, we expect it to return a sequence of strings, and each element is the joined string of the 1-D sequence, but currently, _to_string always returns a string for this case. For example,

>>> from pygmt.alias import _to_string
>>> _to_string([[1, 2], [3, 4]], separator="/", ndim=2)
"['1/2', '3/4']"

The expected output is ['1/2', '3/4'].

This PR fixes the bug.